Electric Tankless questions

I know there isn't much difference in energy savings going from tank to tankless electric heaters. I own a condo that I visit only once or twice a month, so I was thinking it would be a good option. I am only going to install an 18kw 2.3gpm model. I live in Florida, so the groundwater isn't cold. My questions primarily involve brand and gpm.

What brands do you recommend? Here are the ones I am looking into:

-Powerstar A115 (Bosch)
Seems to have bad reviews or mixed reviews

-Stiebel

-Rheem
Not sure if they make it in a size I want

-Ecosmart

I am leaning towards the Ecosmart. It uses standard heating elements that are easily replaced and comes with a good warranty. I have never heard of this brand, however.

My next question involves gpm. Is 2.3gpm sufficient for a whole house system? I'm guessing it would be since some showers only use 1.5gpm shower heads. What type of temperatures can I expect if 2 showers are running at the same time? I believe the ground water is 65 degrees.

Unless you're opting for tankless primarily to gain back a few square feet of floor area moving to an underpowered electric tankless is usually a bad idea. For the money you can buy more efficiency using other methods, and at very low money the standby loss of the tank while your away can be brought to zero by either turning off the breaker (or installing a switch to throw) when you leave. (Turning off both power and the water inputs to any hot water heater is a good idea for any place that sees only intermittent use.)

If you have to upgrade the electrical panel or run a dedicated 75A/240F line to serve the 18kw water heater (which is likely) the payback on reduced energy use would be measured in centuries, and that's assuming you DIDN'T have the option of turning off power to the tank heater when you leave.

But assuming you're going ahead with the project, the simple math on how much power it takes to run two simultaneous 1.5 gpm showers with 65F incoming water:

Shower output is about 105F, so you're looking at a (105-65=) 40F rise.

Divide 60,000BTU/hr by 3412 BTU/kilowatt-hr and you get 17.6 kilowatts.

So you have essentially zero margin on being able to run two simultaneous low flow showers. During the coldest winter months or a cold snap when in coming water temps drop to 60F it wouldn't keep up, but you'd always have enough flow for one.

If somebody decided to start a load of laundry or dishes when someone is in the shower, don't count on it keeping up with those intermittent but high-rate flows either. Scheduling the other moderate to high flow hot water use as to not conflict with showering isn't difficult, but it probably would be necessary. (Been there, done that- I lived with an undersized gas-fired tankless for more than a dozen years in cold-water country.) This might be OK for a weekender spot, but most people would rather not live that way.

One of the main reasons for going to tankless is that i will not have to worry about the heater. I tried turning it on and off, but it is hard to remember each time. Plus, I have guests use the condo as well.

It's your place, you can do what you want with it. But it's an expensive proposition, not a simple drop-in for the tank due to the much higher power draws, which require more electrical work.

Still, 18kw is a bit small for a place with two showers. If the showerheads are 1.6gpm instead of 1.5gpm, or if your line voltage is on the low side you simply can't run two showers without falling behind on temp. A 27kw unit would be enough of an improvement to matter. Since you'll have to be running dedicated power to it whether it's 18kw or 27kw, the modest additional cost of the fatter wire & higher-current breaker(s) somewhat disappears into the total installation cost. But if you don't have enough total power to the panel you may be somewhat stuck anyway.

Another way to get capacity (in showering mode only, not tub fills) without more power is to install a drainwater heat recovery heat exchanger, but this may be difficult or impossible to install in a condo. You need at least 4 ' of vertical drain pipe downstream of the shower(s) to accommodate one of those (probably a long shot here.) A 4" x 48" or 3" x 60" heat exchanger roughly doubles the apparent hot water capacity by pre-heating the cold feed to the shower & water heater with heat extracted from the water going down the drain. With 65F incoming water to the heat exchanger and 100-105 water going down the drain the cold water to the shower is raised to the mid-80s, meaning you end up mixing in more cold-side at the shower, and the water heater is only raising the now lower volume of water 30F (assuming it's set to 115F) instead of 50F. The net result is to about double the apparent hot water flow capacity of the hot water heater while showering.

Stated another way, you use only half the power for the hot water used in the shower. There's no financial payoff in the energy savings in an intermittent use place like this, but it may cost less than upgrading your entire electric service to accommodate a 27kw hot water heater instead of an 18kw heater. They're not cheap- $500-600 (wholesale) but if there's room they're usually pretty easy to install- figure about a grand (maybe a bit more) installed. Bigger is always better, both on diameter and length (more surface area means more heat exchange- really tall ones can exceed 70% heat return, but those rarely fit in residences.) Shorter ones still add capacity, but a 3 x 36 only boosts apparent capacity by ~65-70% (still worth it, if you're marginal, and it's the biggest that fits.)

You could add a relay or a heavy duty switch that turns on the water heater. Its a pretty quick discovery that there is no hot water and only about 15 minutes to get enough to get started. One simply posts a few laminated signs around the cabin telling guests to turn on the water heater.

If you fear they wont shut it off, an electric water heater can be insulated to ANY r-value simply, and your standby losses will be virtually nil.

There are devices that can monitor and control loads like this, so you could say turn it on before you got there so you'd have hot water, and check up on guests to ensure they shut it off. A WH is the least expensive option as an electric WH doesn't really have a big standby loss. It'd be more if the place got really cold, as you'd need to ensure it stayed warm enough not to freeze the supply lines, but you'd have to do that with a tankless as well. May not be an issue with a condo if you have people surrounding you that have their spaces conditioned either way.

Freeze-up inside of even unheated (but insulated) buildings would be an extremely rare situation anywhere in FL, not worth spending the time to think about it. Many homes have gone 50 years without freeze up with exposed pipes in highly ventilated crawlspaces under pier foundations in FL.

Jim- can you point us to any particular phone/internet/other controls to point to that would be suitable for switching water heater sized loads? If readily available this seems like it would be a more cost-effective approach than any tankless based solution.

It's still not clear to me what the motivations are. "One of the main reasons for going to tankless is that i will not have to worry about the heater." If the financial or environmental cost of standby loss is the worry none of the proposed solutions (including tank-wrap insulation) is going to make much sense (but insulating the near-tank plumbing with 5/8"-wall pipe insulation would be cost-effective.) If developing leaks and flooding is the worry, there are cheaper/better solutions (such as isolating the tank with ball-valves, if it isn't already.) I'm not sure what else might be on the worry-list that would drive anyone to considering a tankless. But maybe there's some other concern I'm not thinking of?

The total annual standby loss of a newer electric tank is on the order of 500-700kwh/year, and that's if you keep it at 140F. At the average FL residential retail rate of ~12cents/kwh that's less than $85/year. Even a not-well-insulated beast from the 1980s would have standby losses less than 1000kwh/year- less than $120/year at FL prices. (And spending $50 on a tank-blanket and another $50 for better-grade pipe insulation would be cost-effective, cutting the standby by half for older units. Either is an easy DIY project.)

THere are some X-10 devices designed to control loads of that size and the controller can be connected to the local router. Some can be connected to the phone. Getting more fancy runs into big money, but X-10 is pretty cheap. The succesor to that is Insteon (sp?) that is backwards compatible to X-10. Those often have status feedback, where X-10 generally doesn't. There probably are other methods to do the same thing, but I haven't looked recently.

Unless you're opting for tankless primarily to gain back a few square feet of floor area moving to an underpowered electric tankless is usually a bad idea.

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There are other good reasons. For example, I just converted a ski cabin (previously with a 50 gallon electric tank) to a 28kW electric on-demand. Because the old tank was under the stairs it consumed access to the entire area. It also took hot water over a minute and a half at full flow to reach the kitchen sink. I wasted a lot of electricity (and water) waiting for it to get hot. The new on-demand heater is directly under the kitchen sink and the hot water arrives much sooner with less piping heat loss too. No matter how much insulation you put on the pipes it just isn't that effective. At the same time I installed a drainpipe heat recovery system on the shower using a 6 foot Power Pipe. My electric bills have dropped dramatically. I attribute most of the savings to the relocation of the water heater and the wastewater heat recovery drainpipe. A smaller share of the savings is attributed to the fact that the water heater has zero standby losses while the cabin is unoccupied.

For the money you can buy more efficiency using other methods, and at very low money the standby loss of the tank while your away can be brought to zero by either turning off the breaker (or installing a switch to throw) when you leave. (Turning off both power and the water inputs to any hot water heater is a good idea for any place that sees only intermittent use.)

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While I've turned off water heaters for extended periods of absence, this is not very effective at saving electricity because the hot water still gradually cools and must be brought up to temperature when returning. The heat loss is not stopped, merely slowed down by the gradual reduction in tank temperature. For this to really work one would need to turn off the heater before the last shower and use all that hot water up before it loses it's heat to the environment. And who wants to design their lifestyle around that?

A larger problem with turning off a water heater with a tank is that bacterial blooms can occur as the water drops below 120 degrees F. Often times the residual chlorine is too low to prevent such blooms from occurring.

If you have to upgrade the electrical panel or run a dedicated 75A/240F line to serve the 18kw water heater (which is likely) the payback on reduced energy use would be measured in centuries, and that's assuming you DIDN'T have the option of turning off power to the tank heater when you leave.

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We've already covered why turning off a tank heater is not a safe or effective solution. My ski cabin was built in 1991 and had a 200 amp breaker panel with plenty of room for the 28kW breakers for the on-demand water heater. Cost was minimal because the panel was only 8 feet from the heater location. Because the tank heater was already 20 years old, it needed replacement anyway. I think in my situation payback is in about 5 years. And that's at current energy prices.

My incoming water is only 42-46 degrees F and the 28kW on demand heater has plenty of power for two simultaneous hot showers. If someone else turns on a hot water faucet while two people are showering simultaneously, the flow to the showers is slightly reduced but the temperature stays almost the same and it is a non issue. I recommend getting the type of heater that reduces output volume to maintain a constant output temperature. As an added bonus, the on-demand water heater allows me to change the water in my 380 gallon spa in one go at 4 gallons a minute. Rather than waiting all day for the hot tub to heat up to 104 degrees, I can be using the tub the same evening I changed the water.

People love to talk down the benefits of on-demand water heaters but my situation was tailor made for this conversion and the benefits/savings are obviously substantial. I could not have located a 50 gallon tank under the kitchen sink which is used intermittently all day long while the cabin is occupied. Water also reaches the laundry room, the downstairs shower and the main hand washing sink much more quickly than with the previous tank location. And there are no standby losses on the days the cabin is unoccupied.

Finally, the entire area under the stairs is now accessible and will be converted to a boot drying area and ski storage. Priceless.

"My incoming water is only 42-46 degrees F and the 28kW on demand heater has plenty of power for two simultaneous hot showers."

28kw=~95,000 BTU/hr. With 42F in, 105F out (a reasonable showering temp, far from scalding), is a delta-T of 63F.

That means your max flow is

(95000/63=) ~1510 lbs/hr

which is (1510/60min=) 25lbs/minute

which is (25lbs/8.34lb per gallon= ) 3 gallons per minute. That's enough for ONE full-flow gusher of a shower, and marginally sufficient for two 1.5gpm lowest-flow sipper showers. With 2 gpm shower heads or a sauna-loving skandahoovian taking 110F showers, somebody would get the short-end (of either flow or temperature.)

In other words, without drainwater heat recovery the 28kw unit simply WOULD NOT cut it at two regular shower flows. But since the PowerPipe is returning ~50% of the drain heat to the incoming water stream you're getting ~36kw of water heating, which means it'll handle 4.5gpm of shower flow before it craps out. It's still marginal, but possible with typical showerheads- in the ski cabin application it's essential!

Turning off even fully-hot tanks will still save significant standby power when off-periods are measured in weeks (as with the use pattern indicated in the original post), even though it's pretty useless for overnight or even a 2-3 days intervals. The bacteria issue is a non-starter, when the tank is allowed to stagnate to room temp, as would happen in the type of use indicated. The danger zone for bacteria blooms occurs primarily between 85F-115F for bacteria such as legionella, which it would coast through in well under a week.

The IRC 2012 specs R3 (minimum) pipe insulation on hot water distribution lines, based on very real in-situ measurements- it IS enough to make a difference, but clearly making the distribution runs as short as possible can make a larger difference if it takes 75% of the length out of the system. In a "typical" home with average distribution plumbing lengths and standard hot water heaters, between 15-25% of all hot water heating energy gets abandoned in the distribution plumbing. Moving to a tankless makes that percentage larger, but only because the total input is lower without the standby losses.

If you don't have to upgrade the power service and the distribution lengths are dramatically shortened, with a DIY install there is some payback on an electric tankless, but be sure to include the cost of the drainwater heat recovery in your financial analysis, 'cuz you'd be screwed on 2- shower use without it, and the "free" 14kwh or so during shower flows (probably 50% or more of the hot water use for a ski cabin if you're not refilling the spa every day) is a not-insignificant portion of the total.

Go "high tech" and install a standard tank type heater AND a solenoid valve on the incoming water line. Connect both of these to an accessible toggle switch that the users turn on when they arrive and turn off when they leave. That way there will not be a probem with broken plumbing and the heater will have hot water by the time they are unpacked. Many businesses, such as dental offices, do this also.